Lagging when you pick a contract with big field

[TGA18430]

Hello. I have an issue, when i choose a contract to accept with a large field, my FPS drops instantly to 10-15 nad the game starts lagging. My pc is really not bad. But it happens ONLY if i accept a big field harvesting with a Combine or Beet Harvester. If i choose a contract with a forage harvester then theres no lag and no fps drop. Maybe you know how i could resolve that?